The most commonly used procedure for manufacturing a continuous fiber product such as strand consists of drawing streams of molten material from a bushing of which the base plate is equipped with orificed tips. When cooling means are associated with the tips, stable separation of the streams is enhanced. The tips tend to minimize the phenomenon of "flooding" of the bushing base, particularly at the time of a filament rupture during the fiberization operation. This flooding is manifested by the tendency of the molten material issuing from an orifice to spread over the surrounding undersurface of the base plate, because of capillary action and the wetting effects of the plate. At the moment of filament rupture during the fiberization operation, the presence of tips tends to reduce the massive flooding of the base plate which would otherwise occur.
Taking into account the difficulties in manufacturing a bushing base with tips and also the significant space occupied by the tips, various efforts have been made to eliminate them and to replace the tipped bushing base with a generally flat perforated plate having a large number of orifices within a given surface area. The result of these efforts is the simultaneous attenuation of a larger number of filaments per bushing and consequently the possibility of the manufacture of new products, and a reduction in deformation problems of the bushing base which become progressively more significant as bushing surface area becomes larger.
Nevertheless, the advantage of an increase in the number of orifices per surface unit of the plate is, in practice, rapidly reduced by the flooding problem which occurs at the time of a filament rupture and which is produced much more easily since the distances between adjacent orifices are smaller, and the fiberization conditions, particularly the temperatures adjacent to the orifices of the plate, are less stable or less uniform.
Various different apparatus and processes have been proposed in order to obtain good fiberization stability with these orificed plates, to decrease the number of filament ruptures and the risks of partial or total flooding of the plate, since flooding generally leads to fairly lengthy delays in order to restart the fiberization operation. Various patents, and particularly U.S. Pat. No. 3,905,790, describe a process consisting of blowing a large volume of gas at a relatively high speed, upward, onto a bushing base made of a flat plate of orifices, for the purpose of rapidly cooling the glass cones so as to prevent the flooding, and to eliminate the gases which stagnate on the lower side of the plate and create the hot spots which cause the flooding. In French Pat. No. 2,297,194, the air current blown in the direction of the orifices of the plate is regulated so that the temperature of the latter is at least 28.degree. C. lower than the temperature of the molten glass adjacent to the said plate.
This process, the basis of which is the continuous blowing of air toward the orifices of a flat plate during the normal operation of the bushing, presents certain disadvantages despite the numerous improvements it had brought, disadvantages which are inherent to the blowing itself. In particular, it is difficult to obtain relatively uniform temperatures on the group of orifices and a constant cooling of the glass cones, but these temperature variations on the base of the bushing cause a fiberization instability which unfavorably affects the manufacturing yield. In the event of flooding of the flat plate after filament rupture, the fiberization restarting operations, in general, remain long and complicated procedures. In effect, it is necessary after complete flooding of the bottom of the bushing to increase the gaseous flow to obtain solidification of the glass, to thereafter proceed with alternate reductions and increases in the gaseous flow to gradually reduce flooding. All these operations readily result in thermal imbalance of the bushing, harmful to its proper operation. It should also be noted that this process is all the more difficult to implement when small diameter fibers are desired for manufacture. Furthermore, the fiberization apparatus is fairly complicated due to the presence of the equipment for the supply of gas and of the system for regulating the various parameters relative to the gas blowing.
In an effort to decrease the tendency to flood the bushing, French Pat. No. 2,128,312 describes a process according to which a special gaseous mixture is directed toward a bushing base, whether it is made of a flat plate with orifices or as a plate having parallel, longitudinal channels perforated with a row of orifices. This process consists of continuously emitting an inert or nonoxydizing gas and a hydrocarbon gas, which is decomposed by heat in order to deposit carbon and cause an adsorption of hydrogen on the bushing base. The carbon deposit, which results from the chemical decomposition of the gases in contact with the hot glass and hot bushing base, is supposed to decrease the wetting of the base by the glass, and tends to counteract the joining of the glass cones which are formed at each orifice.
However, this process presents certain disadvantages already mentioned above with regard to the blowing of gas toward the bushing base. In effect, it is difficult to obtain a satisfactory uniformity of the temperatures at the bushing base level and, consequently, the operation is again unstable, specifically for the manufacture of small diameter filaments during which the number of filament ruptures is relatively significant. The complexity of the corresponding apparatus, which comprises perforated or porous tubular parts for the distribution of the gases and an entire supply source system for the inert and the hydrocarbon gases, as well as the character of the gases utilized, prohibitively increase the cost of the fiberization installations.
A process for maufacturing fibers from a flat bushing base perforated with orifices placed in rows between which the cooling fins are situated is disclosed in French Pat. No. 2,014,197, this bushing base being made of special alloys in order to prevent the wetting by the glass. Because of the flat configuration of the bushing base, any temperature differential, whether coming from the glass, the heating conditions or the thermal insulation of the bushing, is reflected at the level of the orifices, with the result that the fiberization operation is very sensitive to these differences and consequently unstable. Furthermore, the proximity of the cooling fins to the bushing base also results in base temperature variations which are detrimental to the proper operation of the process. In the event of partial flooding, the spread of molten glass over the entire bushing base seems unavoidable.
French Pat. No. 1,116,519 discloses the manufacture of glass fibers with an apparatus comprising a bushing and a feeding source of molten glass combined with a rotor equipped with a stop valve. This rotor acts as a regulating pump, to adjust the output rate of glass in the bushing and its pressure on the base, and also serves to homogenize the glass. The diameter of the filaments is modified by varying the speed of the rotor and its vertical position. The bushing base has the general shape of a "V" or of a series of parallel V-shaped elements, at the summit of each of which a row of orifices is provided. This particular design and the placing of the glass under pressure is proposed for the purpose of preventing the flooding. Small cooling fins are present on both sides of the summits of the v-shaped elements.
However, it should be noted that the practical embodiment of this apparatus and its implementation on an industrial scale presents numerous difficulties, especially because of the need to utilize a rotor in the bath of molten glass in order to regulate the glass flow in an effort to inhibit the flooding.
In the following description, the expression "number of orifices per surface unit" or "density of orifices" will be used to designate the relation between the number of orifices distributed in the plate forming the bushing base and the surface occupied by this plate in an average horizontal surface, a surface which is smaller than its surface developed in the apparatus according to the invention.